This invention relates to wood preservative compositions containing copper arsenate and to a method for their production.
2. Brief Description of the Prior Art
Compositions containing, as active ingredients, compounds of copper, of chromium and of arsenic, hereafter called copper/chrome/arsenic compositions, are in widespread use as wood preservatives. Such compositions are required to be water-soluble and are generally made available as pastes, as slurries of solid and solution, or as concentrated solutions, each for dissolution and/or dilution in water before use.
The British Standards Institution has published British Standard No. 4072 relating to the composition of copper/chrome/arsenic compositions and their use in the treatment of wood. This Standard specifies two formulations identified as Type 1 and Type 2 having overall weight compositions as follows:
______________________________________ Type 1 Type 2 ______________________________________ Copper sulphate - expressed as 32.6% 35.0% CuSO.sub.4 5H.sub.2 O Dichromate - expressed as 41.0% 45.0% K.sub.2 Cr.sub.2 O.sub.7 or Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O Arsenic - expressed as As.sub.2 O.sub.5.2H.sub.2 O 26.4% 20.0% ______________________________________
Similar compositions have been Standardised by the American Wood Preservers Association as Types A, B and C having overall compsitions as follows:
______________________________________ Type A Type B Type C ______________________________________ Copper sulphate - expressed as 18.1 19.6 18.5 CuO Sodium dichromate - expressed 65.5 35.3 47.5 as CrO.sub.3 Arsenic - expressed as As.sub.2 O.sub.5 16.4 45.1 34.0 ______________________________________
The above Standards illustrate the range of copper:chromium:arsenic ratios commonly used for wood preservative purposes. The ingredients used to attain such ratios are varied somewhat subject to constraints arising from the avoidance or reduction of the content of undesired impurities, for example the sulphate and/or nitrate anions, in the compositions or arising from the need for water solubility in the composition at least at the final treatment concentration.
U.S. Pat. No. 4,103,000 describes the production of copper/chrome/arsenic wood preservative compositions corresponding to Types A, B and C identified above from copper arsenate and chromic acid and also describes a process for the production of slurries of copper arsenate suitable for such use. The last mentioned process comprises reacting copper metal with arsenic acid at a temperature of 100.degree. C. in the presence of an oxidising agent and of an acid catalyst selected from the group consistng of nitric acid, hydrochloric acid and sulphuric acid. Oxygen is sparged into the reaction mixture, preferably under pressure, as the oxidising agent, although if nitric acid is selected as the catalyst oxygen need not essentially be used.